1. Field of the Invention
This invention relates to cryogenic purification of industrial by-product hydrogen streams to recover a high purity hydrogen product. More particularly, this invention relates to a novel cryogenic purification process which provides increased recovery of purified hydrogen from by-product hydrogen streams, such as those produced in oil refineries and petrochemical plants. The hydrogen thus recovered is sufficiently pure to permit its use in hydrocracking and hydrotreating petroleum feedstocks.
2. Description of the Prior Art
Many methods are known in the art for purifying by-product hydrogen produced in processes carried out at oil refineries, petrochemical plants and like installations, cryogenic methods being perhaps most commonly used. Such prior art cryogenic methods have conventionally involved first combining and compressing some or all of the various hydrogen-containing by-product streams generated during hydrocarbon processing to give a combined feed stream. This combined feed stream is then subjected to a series of heat exchange separations. These separations generally cool the stream no further than is necessary to cool it to a low enough temperature so that sufficient impurities, particularly nitrogen are condensed out to give a purified hydrogen product meeting target purity specifications.
The means known in the art for providing the refrigeration required to carry out such cryogenic purification methods include separate, external refrigeration systems; see, for example U.S. Pat. Nos. 3,626,705, issued Dec. 14, 1971 to Knapp et al and 3,628,340, issued Dec. 21, 1971 to Meisler et al ("Meisler et al I"), achieving a reduction in the pressure of the liquid condensate to cause it to flash at a lower temperature; see, for example, U.S. Pat. No. 3,359,744, issued Dec. 26, 1967 to Bolez et al, and the use of expanders; see, for example U.S. Pat. No. 3,796,059, issued Mar. 12, 1974 to Banikiotes et al. Such simple low temperature flash systems typically remove about 25 percent of the nitrogen contained in the by-product hydrogen stream. Greater nitrogen removal is not possible using such systems, however, since the lower temperatures required to condense additional nitrogen will also solidify the methane in the by-product stream.
Nitrogen and other non-readily condensible impurities, e.g., helium and the like, which have boiling points below that of readily-condensible impurities, e.g., hydrocarbons such as methane, also present in the by-product hydrogen stream are typically contained in by-product hydrogen streams from refinery processes such as fluid bed catalytic cracking of petroleum. If such by-product streams are to be used as the source of hydrogen in typical hydrocracking and hydrotreating processes, all or a preponderance of such non-readily condensible impurities must be removed.
Hydrocracking and hydrotreating are carried out under high pressures, consume large quantities of hydrogen and recycle still larger amounts of hydrogen through the reactor units. Reaction products and readily condensible impurities increase in concentration in the hydrogen recycle stream as the process continues to run, until their equilibrium solubilities in the oils exiting this high pressure loop lead to their removal with these oils. Reaction products and readily condensible impurities may also be removed by solvent scrubbing the recycled hydrogen -- a step which can add significant costs to the process --or by purging a portion of this gas. However, nitrogen and other non-readily condensible impurities, which also increase in concentration in the hydrogen recycle stream as the process continues to run, are poorly soluble in the exiting oils, and thus are not removed with these oils.
The buildup of these non-readily condensible impurities in the loop reduces hydrogen partial pressure until the point is reached at which recycle gas must be purged to reduce the non-readily condensible impurity level. Typically, such a purge will contain about 5 to 10 mol percent nitrogen and at least about 75 mol percent hydrogen. In other words, to purge one mol of nitrogen approximately seven to fifteen times as much hydrogen must also be purged. Reduction of nitrogen and the other non-readily condensible impurities present by purging a portion of the hydrogen recycle stream, although necessary, is quite wasteful of both hydrogen and energy, since the purged gases, compressed under high pressure, are typically vented to a low pressure fuel system.
Hydrogen gas being fed to a hydrocracking or hydrotreating plant typically should contain not more than about 1.5 mole percent of non-readily condensible impurities if problems such as those mentioned above are to be avoided. Prior art cryogenic purification processes used to achieve this result generally have operated in one of two fashions.
In the first, all of the by-product hydrogen streams generated during hydrocarbon processing, those containing non-readily condensible impurities as well as those in which readily condensible impurities predominate, are first combined into one feed stream containing hydrogen, various hydrocarbons, and non-readily condensible impurities, including nitrogen; see, for example, U.S. Pat. No. 3,691,779, issued Sept. 19, 1972 to Meisler et al ("Meisler et al II)". If, however, all these by-product hydrogen streams are combined, expedients such as colder condensation temperatures, or a system which will adsorb the non-readily condensible impurities, or both must be employed to produce suitably purified hydrogen, with consequent increased energy consumption and capital costs. In the process of Meisler et al II, for example, an adsorption system is employed to remove the nitrogen remaining in the combined feed stream after this stream has passed through a series of cooling and condensation stages conducted at successively lower temperatures.
Another expedient which has been used in purifying such combined feed streams is to wash the feed stream with liquid methane*; see, for example, Eugene Guccione, "Cryogenic Washing Scrubs Hydrogen for Liquid-Fueled Rockets," Chemical Engineering 70, May 13, 1963, pp. 150-152; Wolfgang Forg, "Purification of Hydrogen by Means of Low Temperatures," Linde Report on Science and Technology, 1970. Since this requires a methane still, a pump and several heat exchangers to remove nitrogen and carbon monoxide from the circulating methane, it too is a relatively expensive system to operate.
The second type of prior art cryogenic processes for providing purified hydrogen gas feeds to hydrocracking or hydrotreating plants in essence involve giving up on recovering high purity hydrogen from the non-readily condensible impurity-containing by-product streams. Instead of treating all of the by-product hydrogen streams from a hydrocarbon processing unit, the readily condensible impurity-containing streams are purified while the non-readily condensible impurity-containing streams are discarded or merely used as fuel gas.
The Bolez et al U.S. Pat. No. 3,359,744 provides an example of such processes. It discloses injecting part of its purified hydrogen product into flashed impure liquid condensate obtained, as was the purified product, from by-product hydrogen streams containing readily condensible hydrocarbon impurities. This injection provides additional refrigeration to lower the partial pressure, and thus the temperature, of the hydrocarbon impurities present, and produces higher purity hydrogen. This result is accompanied, however, by significant losses of purified hydrogen product used to inject the flashed impure liquid condensate.
U.S. Pat. No. 4,242,875, issued Jan. 6, 1981 to Schaefer and of common assignment herewith, discloses a cryogenic purification process for purifying by-product hydrogen streams in which the streams containing substantially only hydrocarbons as impurities are kept separate from streams containing non-readily condensible impurities. In particular, two separate by-product gas streams containing hydrogen in recoverable amounts, one of which contains non-readily condensible impurities having a boiling point below that of methane, are passed through a successive series of cooling and separation stages. At each separation stage, a liquid bottom fraction containing hydrocarbons is separated from the overhead of the respective by-product feed gas stream until the overhead of the hydrogen product feed stream attains the desired purity. The hydrogen product overhead is passed back through the heat exchange means to provide refrigeration for the process, and the overhead is recovered as product. The overhead of the feed stream containing the non-readily condensible impurities is injected into the liquid condensate stream containing the combined liquid bottom fractions. This reduces the partial pressure of the condensates, thereby reducing their temperature. The condensate stream is also passed back through the first and second heat exchange means to provide increased refrigeration for the process, and the condensates are recovered as a fuel gas by-product; see column 3, lines 11-32 of the Schaefer patent. This process is not designed to maximize hydrogen recovery from all the by-product hydrogen streams it treats.
The need exists, therefore, for a cryogenic process that can be used to purify by-product hydrogen streams containing non-readily condensible impurities as well as those in which relatively condensible impurities predominate, without additional costly purification steps and without the sacrifice of an of the purified hydrogen produced.
It is an object of this invention to provide a cryogenic process for purifying industrial by-product gas streams containing recoverable amounts of hydrogen, such as those produced in oil refineries and petrochemical plants, which accomplishes increased recovery of high purity hydrogen.
It is also an object of this invention to provide a cryogenic process for purifying industrial by-product hydrogen gas streams, including those containing non-readily condensible impurities having boiling points below that of methane, which accomplishes increased recovery of high purity hydrogen.
Another object of this invention is to provide a cryogenic process for purifying industrial by-product hydrogen gas streams, including those containing non-readily condensible impurities having boiling points below that of methane, without the need for additional separation stages to remove such impurities and without the sacrifice of any of the high purity hydrogen produced.
A further object of this invention is to provide hydrogen which has been sufficiently purified to permit its use in hydrocracking and hydrotreating petroleum feedstocks.
These and other objects, as well as the nature, scope and utilization of the invention, will become readily apparent to those skilled in the art from the following description, the drawing, and the appended claims.